Lithography is a key factor in the drive for higher levels of microcircuit integration. Dynamic RAMs have quadrupled in the level of integration every three years as a result of the reduction in minimum geometries and increases in chip size. As minimum geometries become smaller, alternatives to optical lithography, such as electron beam direct write, X-ray and electron/ion beam proximity technologies become attractive. The latter three technologies are still in their infancy relative to optical lithography and still have obstacles to overcome, including decreased throughput, low source brightness and mask complexity, respectively.
While optical lithography continues to be the dominant technology because it is well established and is capable of implementing sub-micron resolution at least as low as 0.35 micron, efforts into attaining smaller geometries are looking toward the newer technologies. In both optical lithography and its alternatives, progress into the realm of shorter wavelengths introduces increased sensitivities to minute surface imperfections including contaminants on optical surfaces, aberrations introduced by lenses and mirrors, photoresist thickness and surface variations and wafer flatness and planarity.
Due to manufacturing limitations most optical elements have plane, spherical or paraboloidal form. The restriction to surfaces of simple form imposes limitations in an optical system's ability to realize diffraction-limited performance due to optical aberrations.
In optical machine vision devices great attention is usually given to the quality of the optics between the observed target and detector. In semiconductor metrology this is particularly true as the optics is expected to have as little error contribution to the measured signal as possible. In the optics optimization process, standard optics analysis methods are usually employed where an ideal simulated ray cone is launched from points on a target towards a detector while the intermediate optics are improved to achieve the best converging ray cone at the detector. The best possible cone at the detector will produce a diffraction limited Airy disk, which is a diffraction pattern resulting from a uniformly-illuminated circular aperture having a bright region in the center.
The point spread function (PSF) describes the response of an imaging system to a point source or point object. The degree of spreading (blurring) of the point object in the image is a measure for the quality of an imaging system. Reduced quality cone can be described with a Point Spread Function (PSF), i.e., the converging cone quality is limited by the optics aberrations. It is assumed that illumination optics is perfect and that the ray bundle emanating from the target, as a result of illumination reflection, towards the detector is also perfect, i.e., suffers no aberration or diffraction at its origin. While this is an excellent assumption in many cases, it falls short when measuring deeply sub-wavelength features. Traditional methods for improving illumination include illumination optics optimization and the use of static and dynamic diffusers. However, improved illumination optics cannot always compensate for imperfect light source and the use of diffusers relies on their random nature, thus, support limited control over illumination quality. Here a dynamic method is proposed which includes 2 structured elements, one in field conjugate and one in pupil conjugate which enables control on illumination spatial and angular characteristics.
An important metrology technique used in semiconductor manufacturing is the measurement of overlay error between successive patterned layers on a wafer. Overlay metrology is one of the most critical process control techniques used in the manufacturing of integrated circuits and devices. Overlay accuracy generally pertains to the determination of how accurately a first patterned layer aligns with respect to a second patterned layer disposed above or below it and to the determination of how accurately a first pattern aligns with respect to a second pattern disposed on the same layer. Presently, overlay measurements are performed via test patterns that are printed together with layers of the wafer. The images of these test patterns are captured via an imaging tool and an analysis algorithm is used to calculate the relative displacement of the patterns from the captured images.
It is within this context that embodiments of the present invention arise.